The user edit page redirected to its ?next= parameter after saving,
whatever it held: a link to the edit page with next=https://other.site
sent the administrator who saved the form to that site (CodeQL
py/url-redirection). The account page links there with a next of this
site.
next is now followed only when url_has_allowed_host_and_scheme accepts
it for this host, as user_instance_delete already does; otherwise the
user list. The test fails on the previous code for an absolute URL, a
scheme-relative one and a backslash variant.
